A prior method for determining optical quality of a transparent product, such as a front windshield for a motor vehicle, involves projecting an array of dots through the product and onto a screen. The dots are then measured manually, and the measurements are compared with predetermined, standard measurements to evaluate optical distortion of the product.
Because the dots are measured manually, this method is rather time-consuming, fatiguing and error prone. Furthermore, the method provides limited coverage of the product. For example, distortion occurring between dots is not detected.
Another method for determining optical quality of a transparent product involves using a test pattern comprising a plurality of diamonds or equivalent pattern. A moveable light source is positioned behind the test pattern, and its movement is synchronized with a moveable line-scan camera positioned in front of the test pattern. As the light source and camera move with respect to the product, the camera views images of the test pattern as seen through the product. Measurements of diamonds detected in the images are compared with undistorted diamonds to evaluate distortion of the product. This method also provides limited coverage of the product. Furthermore, the apparatus used to practice the method is relatively slow, complex and costly.